The concept of ‘seeing sound’ might seem preposterous, but when you consider that sound is just compressed air travelling in waves, the idea becomes a bit more logical. The real trick is capturing air flow in a way that makes it perceptible to the human eye. Through some clever tricks, scientists are able to do just that using a technique called Schlieren flow visualisation, which is used to study everything from heat and sound to aerodynamics and the spread of disease.